docs: def _addAuthLevelAlias(self, auth_level_uri, alias=None):
          """Add an auth level URI alias to this request.
  
          @param auth_level_uri: The auth level URI to send in the
              request.
  
          @param alias: The namespace alias to use for this auth level
              in this message. May be None if the alias is not
              important.
          """
          if alias is None:
              try:
                  alias = self._getAlias(auth_level_uri)
              except KeyError:
                  alias = self._generateAlias()
          else:
              existing_uri = self.auth_level_aliases.get(alias)
              if existing_uri is not None and existing_uri != auth_level_uri:
                  raise KeyError('Attempting to redefine alias %r from %r to %r',
                                 alias, existing_uri, auth_level_uri)
  
          self.auth_level_aliases[alias] = auth_level_uri